Key Points:

World leaders will converge on Bali today for the start of negotiations which experts say could be the last chance to save the Earth from catastrophic climate change.

The United Nations conference of 191 countries is the beginning of talks to agree on a new international treaty to cut greenhouse gas emissions causing climate change and continue the work of the Kyoto Protocol after 2012.

Last week negotiators were "managing expectations" and warning that the aim of the gathering of 10,000 people was to agree to a "road map" or timetable for negotiations.

But even such modest ambitions are likely to generate controversy over key issues such as which countries will be expected to sign up to targets to cut emissions, how cuts can be made and who will pay.

Key figures also warned that what is agreed must lead to an agreement at another conference in two years.

That would give Governments time to ratify the new treaty before the first phase of Kyoto expires. Crucially, this is after the next United States president is installed - raising hopes that the biggest obstacle to agreement will have been removed.

Privately, officials admit some extension might be needed to the self-imposed 2009 deadline but publicly all sides appear to have agreed to use it to pile the pressure on negotiators during the 12-day conference.

"[The new agreement] has to be in 2009 because we know Kyoto expires in 2012, and it can take a year or two for Governments to ratify, so 2009 really is the cut-off point," said Nick Nuttall, the official spokesman for the UN Environment Programme.

Negotiations will reach the crunch in the final three days, when senior figures arrive to finalise any agreement.

The first issue to be decided will be the promise to limit global warming to an average temperature increase of 2C above pre-industrial levels - and what cuts will be needed to meet that.

Estimates range from 50 to 80 per cent by 2050.

With the rapid growth of big economies such as China, India and Brazil, the major debate will be over whether developing nations are forced to accept mandatory targets to cut emissions. Last week China and Brazil were reported as saying they would only make cuts if they were paid.

The other key area of controversy will be how emissions reductions can be made. Likely flashpoints include whether countries should be paid not to chop down forests, support for nuclear power and biofuels as "low carbon" sources of power.

There will also be pressure from some quarters to set specific targets for some sectors, for example buildings and transport, and to include emissions from aviation and international shipping in future agreements.

Hopes of a successful conference are high, partly because countries are not expected to sign up to any specific commitments. The meeting follows a spate of high-profile reports warning of the impacts of climate change.

"Bali could be the last chance to avoid the worst effect of global warming," said Tony Juniper, executive director of Friends of the Earth.

"The politics of this are changing very quickly. One should be cautiously optimistic about the prospects for getting the outline of the deal because we're stuffed if we don't."

- Observer